It is common today to manufacture parts with closed loop numerically controlled machining systems. Those machining systems automatically control the positioning and operation of a machine tool with respect to a workpiece to machine the workpiece to precise tolerance in accordance with a predetermined computer program.
In addition to automatically controlling the actual machining operation, it has been proposed to automatically calibrate the components of the machining system to improve and maintain machining system accuracy and workpiece tolerances. Such calibration ideally should be accomplished on the machining system without human intervention as one step in achieving a completely automated production facility.
Barlow et al. U.S. Pat. No. 4,382,215 ("the Barlow patent") describes a system to automatically machining a workpiece to precise dimensions under computer numerical control. The machining system includes a movable probe mounted in a turret containing a plurality of tools. The turret can be rotated to bring selected tools into position for machining a workpiece mounted in a chuck fixed to a rotating spindle. The movable probe first is calibrated against position reference surfaces on the chuck. The calibrated movable probe then is used to calibrate a fixed tool probe mounted to a stationary support for the machining apparatus at a location remote from the area where workpieces are to be machined. The calibrated fixed probe next is used to calibrate the cutting edges of the tools in the turret. These calibration procedures are carried out under closed loop control. Tool position offsets are automatically determined in light of these calibration procedures. Those offsets are used to modify the machining operation programmed into the computer numerical control to improve machining accuracy.
Although the machining apparatus of the Barlow patent is a significant advance in achieving a totally automated machining apparatus and operates well for its described purposes, Applicant has been able to develop a significant improvement on such apparatus. Applicant has unexpectedly discovered several problems regarding the location and mounting of the fixed tool probe in machining apparatus similar to that of the Barlow patent.
In most automated machining apparatus, there is not enough room to mount the tool probe in the location suggested by the Barlow patent because this location is occupied by auxiliary equipment such as a mechanism for collecting the scrap material machined from the workpiece.
The simplest arrangement from a computer programming standpoint is to have the two probe axes oriented parallel to one of the x- or z-axes of the machine during a calibration operation, as shown in the Barlow patent. However, in many machining apparatus, the axis of rotation of the turret is at an angle with respect to both the x- and z-axes of the machine. Accordingly, the movable probe in such a turret is oriented at an angle with respect to both the x- and z-axes of the machine when the turret is indexed to bring the movable probe into position for contacting the fixed tool probe. The point at which the movable probe emits an output signal when it contacts the fixed probe is a function of the angle of the movable probe with respect to the x- and z-axes. If this angle is not taken into account, an error in the machine calibration can result. This angle can be taken into account in the computer program to eliminate such error, but this introduces additional programming complexity which could compromise speed, reliability, and accuracy. Thus, it is difficult to use the Barlow arrangement of probes in a machining apparatus having a turret axis that is not parallel to the x- or z-axes of that machining apparatus.
Another problem with the location of the fixed probe in the Barlow patent is that machining and workpiece measuring take place at one end of the machine and tool calibration takes place at the other end of the machine, a substantial distance away from the machining and measuring operations. Movement of the turret over this distance adds additional unwanted inaccuracy due to cummulative positioning errors, such as those caused by backlash in the mechanical connections between the drive motors and the turret and in the mechanical connections between the drive motors and the position sensors.
Accordingly, there is a need for a tool probe in the vicinity of the machining environment. This causes an additional problem because the machining environment is a hostile one. Cooling and lubricating fluid is sprayed onto the moving parts and much heat debris are generated during machining thus limiting the accuracy, reliability, and useful life of any tool probe in such an environment.
One approach which was tried to overcome these problems was a separate mechanism which supported the tool probe at one end of a swing arm. The swing arm was constructed such that it was able to move the probe into the machining area when it was desired to perform a calibration operation with the tool probe. At all other times, the swing arm positioned the tool probe outside the machining environment until needed for a calibration operation. It was found that such an arrangement was not rigid enough to provide accurate calibration. The cost of strengthening the swing arm to provide adequate rigidity was too prohibitive. The additional swing arm also resulted in a more complicated mechanical arrangement which took up too much space and resulted in additional complexity in the computer program.
Another approach was to remove the workpiece from the machine tool and replace it on the spindle with a tool probe when it was desired to perform a calibration operation with the tool probe. This was also unsatisfactory because either a machine tool operator had to manually fix the tool probe to the spindle or some form of swing arm had to be devised to do the same. Manually manipulating the tool probe of course prevented the achievement of a completely automated machining operation and the swing arm would introduce the mechanical and electrical complexity described above. In addition, removal of the workpiece during a machining operation introduced further error.
An additional technique of solving the problems associated with locating a tool probe in an automated machining apparatus was tried. The tool probe was mounted on the tool changing mechanism. This tool changing mechanism comprises a swing arm with a pair of jaws at one end for grasping a tool. The swing arm is able to remove tools from a drum containing a number of tools and place those tools into the turret of the machining apparatus. The swing arm also removes tools from the turret and replaces them in the drum. The rigidity of this arrangement also proved to be inadequate, especially for machining jet engine parts, some which require dimensional tolerances as low as 0.001 inches.
Accordingly, a novel mounting of a calibration means has been devised which avoids the problems associated with prior arrangements.